I should also add that I've now tried this method:

<?php
$random_imgs = array('header_bg.jpg', 'header_bg2.jpg');
$random_img = $random_imgs[rand(0, 1)];
?>
<img src="includes/templates/santa/images/<?php echo $random_img; ?>" width="369" height="176" alt="Welcome to FairytaleMail's Christmas Pages!" /></div>

</div>
if I change the <?php echo $random_img; ?> and replace it with header_bg.jpg, I see the image; the filepath works.

It seems that whenever I attempt to use any kind of script to randomise the images, zencart stops me. I've determined that it is a permissions issue, but I don't have much experience with these, and don't know where to start